The present invention relates to an optical attenuator, and more particularly, to an optical attenuator of which the optical fibers have sections formed into non-spherical lenses to reduce back-reflection.
Generally in an optical transmission system, an optical signal is output with high power and attenuated by an optical attenuator for proper use in a receiver side. Such an optical attenuator for attenuating a high-power optical signal may be used for adjustment, correction, and various measurements of an optical communications system, or compensation for local losses of an optical fiber transmission line. It can also find its applications in various optical devices.
Many kinds of optical attenuators have been developed for such various applications, of which one type is illustrated in FIGS. 1A and 1B.
Referring to FIG. 1A, to attenuate an optical signal, a conventional optical attenuator has two optical fibers spaced by a predetermined distance d1 and an air layer 4 formed between both the optical fibers. That is, in this optical attenuator, an optical fiber inputting unit including a first optical fiber 1 and a first supporting holder 2 is detached from an optical fiber outputting unit including a second optical fiber 5 and a second supporting holder 6 by the predetermined distance d1, and an air layer 4, i.e., an air gap intervenes between them. Thus, the optical fiber inputting and outputting units are arranged with the air layer 4 therebetween in a sleeve 9 of a predetermined shape.
In such an optical attenuator as constituted above, since an optical signal emitted from the first optical fiber 1 is diverged in the air layer 4, only part of the optical signal impinges on the second optical fiber 5, thus resulting in optical attenuation. Here, Fresnel reflection, namely, back-reflection occurs at the boundary surface of the first optical fiber 1 and the air layer 4, causing a reflection loss of about 14.4 dB. Thus, the optical attenuator has limitations for use in a practical optical system. Fresnel reflection, i.e., back-reflection indicates the phenomenon in which when an optical signal passes through two materials of different refractive indices, it is partially reflected at the boundary surface of both the materials. As shown in FIG. 1B, for example, assuming that when an optical signal P.sub.o is vertically incident from a glass into the air, the amount of an optical signal reflected from their boundary surface, that is, the amount of back-reflection, is P.sub.r, the amount of a transmitted optical signal is P.sub.t, the refractive index n1 of the glass is 1.47, and the refractive index of the air is 1, the reflexibility R of the vertically incident light is calculated by ##EQU1##
Here, the reflection loss P.sub.r of the reflected optical signal is calculated by EQU P.sub.r =-10log.sub.10 R=14.4 dB
As a result, the optical attenuator having the air layer between the two optical fibers shows the drawback of a reflection loss of 14.4 dB or above due to Fresnel reflection, and thus a great likelihood of causing serious errors in a light source of a practical optical system.
FIGS. 2A and 2B illustrate an example of an optical attenuator which reduces back-reflection caused by Fresnel reflection from the vertical section of an optical fiber. The optical attenuator of FIG. 2A is disclosed in Japanese Laid-Open Patent Publication Showha 59-94702. This optical attenuator performs optical attenuation with two optical fibers and optical supporting holders having inclined portions. It has an optical fiber inputting unit including a first optical fiber 12 and a first supporting holder 13, and an optical fiber outputting unit including a second optical fiber 16 and a second supporting holder 15, and an optical attenuating filter 14 having a predetermined thickness d2 inserted between the optical inputting and outputting units, all of which are arranged in a predetermined sleeve 19.
In this optical attenuator, an optical signal emitted from the first optical fiber 12 is attenuated in the optical attenuating filter 14 and reaches the second optical fiber 16. To reduce back-reflection of the optical signal, the first optical fiber 12, the first supporting holder 13, the second optical fiber 16, and the second supporting holder 15 are ground to have slopes, respectively. That is, referring to FIG. 2B, when light A travelling through the first optical fiber is emitted from the inclined section of the first optical fiber, part of the optical signal becomes transmitted light B. and the rest thereof becomes reflected light C at the inclined section. The reflected light C travels relying on a reflection rule, as shown, and thus does not interfere with the travelling light A. Therefore, with an end portion of the optical fiber formed into a slope, the amount of back-reflection caused by Fresnel reflection or back-reflection in the optical fiber having a vertical section of FIG. 1A can be reduced.
However, such an optical attenuator having inclined sections as described above is costly due to many processes accompanying the grinding of the inclined sections.